Multiplex compressional wave system



Oct. 1, I946. w. P. MASON MULTIPLEX COMPRESSIONAL WAVE SYSTEM Filed Oct. 24, 1942 4; Sheets-Sheet l lNl/ENTOR W I? MASON Arrow/Ex zmsgss Oct. 1, 1946.

W. P. MASON MULTIPLEX COMPRESSIONAL WAVE SYSTEM 4 Sheets$heet 2 I Filed Oct. 24, 1942 Y .uk hb Nb INVENTORY M. P MASON BV z ATTORNEY 24, 1942 4 sheets-sheet 4 Filed Oct.

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L'GUAL AREA GRATING SQUARE LAW GRAT/NG v. mfl E f N m 5 W R0 m a m T W Mr M W w. 2 0 ml w- 8 F 4 Patented Oct. 1, 1946 MULTIPLEX COMPRES SIONAL WAVE SYSTEM Warren P. Mason, West Orange, N. 3., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application October 24, 1942, Serial No. 463,279

This invention relates to multiplex channel transmission systems and more particularly to multiplex compressional wave systems employing diffraction gratings in compressional wave filters for separating the energies of the various channels.

An object of the invention is to improve the selectivity attainable in wave filters at relatively high frequencies.

Another object is to increase the frequency limits of effective compressional wave filters.

A further object of the invention is to decrease the number of elements of a diffraction grating required to give a definite discrimination.

Still another object of the invention is to increase the major lobe directivity of a diffraction grating filter with reference to subsidiary or minor lobes.

Diffraction gratings are well known especially in the field of optics. They depend upon the fact that a plane wave front may be broken up into narrow striations which act as new sources of waves that reinforce each other in definite directions, depending upon the spacing of the sources and the wave-length of the energy. Accordingly, as the wave-length varies the direction of reinforcement for a given grating also varies with frequency with the result that incident waves of a wide band of frequencies are broken up by the grating into components dispersed at different angles. It is, therefore, possible by means of a diffraction grating to separate a wave of one frequency or of one band of frequencies from one of difierent frequency characteristics.

Two types of difiraction gratings have been used, In the reflection grating parallel bars having reflecting surfaces and separated by non-reflecting absorbing areas return the wave energies to reinforce each other along lines on the same side of the grating as the source. The other .type of diffraction grating, the transmission grating, comprises parallel energy-absorbing or reflecting bars and intervening slots through which the wave energies pass to reinforce each other along lines on the opposite side of the grating. These lines at which the reinforcement occurs lie at angles with respect to the plane of the grating face which depend upon the wave frequency.

In both types of grating, the distance between the centers of the elements whether reflecting elements or transmission elements or absorption elements is uniform. Assuming elements of equal lengths, the areas of wave energy utilized, that is, the areas of the slots in the case of the transmission type .or the areas of the reflecting bars in 11 Claims. (Cl. 178-44) the case of the reflecting type are, in the optimum condition for maximum effect, equal to the areas not utilized. In other words, in the transmission grating maximum efiects are obtained when the slots have faces which are equal in width to the faces of the intervening bars. In the reflection grating maximum efiects are obtained when the widths of the reflecting surfaces of the bars are each equal to the widths of the intervening absorbing strips. Such diffraction gratings cause wave energy of a given frequency to reinforce in several directions, the angles of which with reference to the plane of the energy exit face of the transmission grating may be simply expressed by:

where A is the wave-length of the energy in the medium beyond the grating and d is the distance between centers of slots, that is, the separation of the virtual sources. Y

The effect of the series of dispersion angles for each frequency is to produce a first order diffraction lobe and higher order diffraction lobes for each individual Wave-length. Each order of diffraction tends to present a major lobe and several adjacent minor lobes. It follows that there are a number of angles at which any one of the frequencies appears so that there is a possibility of overlap of a major lobe of a wave of one frequency with a minor lobe of another frequency. If compressional wave responsive devices, which are non-selective as to frequency, are to be used, it is obviously desirable to reduce the minor lobes as much as possible so as to increase the amplitude differentiation between these unwanted lobes and the desired major lobes. Applicant has discovered that by varying areas of the grating elecoscos- 2} etc.

ments in such manner that a larger percentage of the wave energy is derived from a central element and a smaller percentage from a marginal element, the directivity of the grating for a particular frequency may be improved in the sense that the ratio of the response at the major lobe to that obtainable in other directions is increased. Moreover, the absolute energy of the major lobe is not substantially diminished so that the gain in discrimination is attained principally by reduction of minor lobes. Various relationships of the grating areas may be employed but in each the area of the central element should be limited, as has already been suggested, to a magnitude not greaterthan the area of the contiguous void elements. According to one relationship which has been found advantageous the widths of the elements may vary in accordance with the square law so that the widths of the individual elements increase passing from the outer element toward the center in accordance with the square of their distances from the outer margin of the grating.

Diffraction gratings of the type described be employed to differently diffract the individual components of a wide band of compressional waves. If the beam of waves, as a whole, encompasses a considerable cross section area overlap of the difierently refracted beams may be markedly reduced by the use of a converging lens. The individual frequency or the subband frequency beams so separated and fccu'ssed at dif ferent positions may then be impressed upon compressional wave responsive devices placed at the respective foci. This, therefore, afiords a means of effectively separating the different frequency components of a compressional wave band consisting of extremely short waves.

The invention m'ay-bebest understood by reference to the following detailed specification taken in connection with the accompanying-drawingsin which:

Fig. l: is a diagram to assist in explaining the principles of the invention;

Fig; 2 illustrates diagrammatically the circuit and apparatus of multiplex carrier wave system employing compressional waves; e

Fig. 3 illustrates on an enlarged scale the designof: a reflection type diffraction grating employed in the system of Fig. 2; r r

Fig. 4 illustrates a modification of the system of Fig; 21in which a transmission type diffraction grating is employed;

Fig. 5 illustrates on an enlarged scale details of the diffraction grating of Fig. 4;

Fig; dillustrates a'modification of the grating ofFig ;5;- r r Fig. -'7 i a graph of directional response of a compressional wave filter system showing'the ii..- provement attained by the use of onespccies of the invention;

Fig. 8 is a similar graph showing the result obtained' with the use of another modification; and

9 illustratesa systemin which aconcave reflect-ion gratingreplacesboth the plane grating andthe converging-lens of-theprecedingsystems. Referring to Fig: 1 let b1 and barepresent t-wo proximate reflecting bars 01 a di-firaction grating the centers or and c2 of whichare spaceda distance d.- Two rays 81 and S2 rom a plane wave front source of wave-length are incident simultaneouslyupon the two bars at their central points. Assuming that or be considered as a new point source, its energy at one cycle laterwill be spread out over a circlewhosecenteris or and Whose radius is A. Accordingly, a line from 02 passing tangent to the circle as' at P will indicate a locus of equalphaseelrects from the two centers cz-and c1. Denoting by ri-the angle'which theray crP makes with reference to the plane of the grating b1, 192 one obtains the well-known'relationship lobe of the diffracted beam. 'By asimilar reasoning the relationships A cos 6 2X 1 =cos 0: (3) and =cos 0,1 (4) may be ascertained to indicate the directions of the second and third order lobes, respectively. It will be apparent therefore that the wave front of the first order diffraction will be directed as is indicated by the ray S3 connecting points or and P the wave fronts of succeeding orders of diiiraction will be oriented in more upward directions in Fig. l, in accordance with the magnitudes of 92 and 93.

The analysis presented is for the case in which the diffraction grating has a plane facial surface perpendicular to the incident rays. In that instance the directly reflected rays 84 and S5 are returned along the paths of incident rays S1 and S2. It will be apparent, therefore, that the angular separation between the directly reflected rays which include components of all frequencies and the first order diifracted beam for any particular frequency or narrow bane of frequencies is meas ured by 90 0. That separation may be made as great as desired by suitably relating d and If it be desired for any reason to orient the grating at a dififerent angle than the perpendicular position to incident rays the directly reflected rays corresponding to S4 and S5 will of course return along parallel paths determined by the well-known principle that the angle I of reflection is equal to the angle of incidence. The diffracted beams will be sent out from the diffraction grating at still different angles determined by theprinciplepresented in. the analysis of Fig. 1 that the energies from the various new centers are in phase coincidence in a particular direction. The direction of the first order of diffraction will accordingly be separated from that of. the directly reflected beam by an angle 81 so that with ref erence to the plane of the grating face the angle of the beam of the first order of diffraction will be [QT-(@3801, that of the second order diffraction beam [90(sifi2) etc.

Referring to Fig. 2, T1, T2, 'I's, T4 illustrate, respectively, four individual transmitting channels of a multiplex carrier wave system. As indicated these channels may each extend over a. frequency range of 3 kilocycles, the four channels as a whole lying, withinthe band of, 50 to kilocycles. Th four transmitters, T1, T2, T3, and. T4 may be connected in series to a transmission linel terminating at a remote station in an electric wave to sound Wave transducer 4 having a diaphragm or sound radiating eiement 5 from which emanates a beam of high frequency. compressional Waves corresponding. in frequency and. in their modulation to the electric wave received over the transmission line I. The beam of compressional waves produced by the diaphragm. 5 and including components of the various frequencies of the electric waves transmitted over. the line I is permitted to fall upon a reflection type diffraction grating 6 positioned in the path of the beam. The diffraction grating 6, the structure of which will be. explained subsequently, serves to diflract the mid-frequency components f1, f2, f3, and f4 of the four bands at the dilTerent angles indicated in the drawings. In the path of. the diffracted beams is placed: a converging lens 1' ofplaneconcave type andwhich may be either cylindrical or spherical depending on whether it is desired to focus the beams alonglines or at points. The lens I may consist of any suitable homogeneous solid material and preferably of some plastic material such as Tenite II (cellulose acetate butyr ate) isobutyl methacrylate, or vinyl chloride. At the -respective fociof the four bands are.posi tioned compressional wave responsive devices R1,

R2 R3. and R4. The devices R1, R2, R3 and R4 are illustrated diagrammatically as of the piezoelectric type and each may be associated as is shown in the case and indicating apparatus. Device R4 is, for example, connected by an electric circuit 8 to the input terminals of an amplifier 9, the output of which is connected to a demodulator which in turn supplies speech or other low frequency signals such as originated at transmitter T4, to the signal indicator H, which may be a telephone receiver, a loud-speaker or any suitable signal indicating or recording appliance. It will therefore be apparent that the four individual messagestransmitted over the line I as modulated carrier current may be separated and supplied to individual terminal circuits by the mechanical wave selecting system comprising sound producer 5, difiraction grating 6, compressional wave lens 1. and compressional wave responsive devices R1, R2, R3, and R4.

The beams of frequency f1, f2, f3, f4 have been dealt with on the basis of the mid-band frequency of each beam. Since each band encompasses a range of 3000 cycles the limiting frequency rays will diverge slightly from the central frequency range, but the amount of divergence over distances which are not excessive will not cause too great a departure from the position of the mid-band frequency ray to substantially affeet the operation of the system.

The transverse dimensions of the sound wave source 5 for effective directive transmission should be relatively large compared with a wave length of the emitted energy. The diffraction grating should therefore encompass a space great enough to receive efliciently substantially all of the directive beam from the diaphragm. It follows that the supersonic beam will have a fairly large cross section in space. This situation is indicated in Fig.2 in which the marginal rays of the bands 11, f2, f3, f4 are indicated. The lens 1 should be given such dimensions and should be so placed with reference to the grating that the foci at which the bands respectively converge may be sufficiently separated to enable the compressional wave responsive devices R1, R2, R3, R4 to be given practicable dimensions and so insure that the zone which each responsive device occupies is reasonably free from energies of the unwanted bands. The orientation of the reflection grating performs an additional function in enabling the compressional wave receiving apparatus to be placed on the same side of the difiraction grating as is necessary in the case of the reflection type without interfering in position with the compressional wave beam emitter 4, 5 or being directly affected thereby. If necessary a sound bafile [9 may be interposed to prevent transmission directly from diaphragm 5 to compressional wave receiving devices R1, R2, R3, R4. In general, however, these compressional wave devices are preferably made as directive as possible with respect to their receptivity so as to reduce the efiect of waves from extraneous sources. Fig. 3 shows in more detail the structure of part .of the reflection type difiraction grating 6. As illustrated, the grating consists of alternate compressional wave reflecting bars and intermediate compressional wave absorbing strips. The bars may, for example, consist of highly polished steel. For most efficient reflection, the steel bars should have a thickness of about one-quarter wavelength. .Although for convenience in illustration a much fewer number of bars and absorbing strips of R; with suitable translating is shown in Fig. 3, in an actual grating there should be preferably as many as fifty reflecting bars and a structure of two hundred bars is more efiective. The central reflecting bar 12 may have a width about equal to that of the adjacent absorbing strips [3 and I4. Succeeding bars should be so positioned that their centers are at a distance of approximately 11 from the center of the nearest bar. Their widths, however, progressively decrease in accordance with a square law distribution so that beginning with the outermost bar I8 which may have an area A, the area of the-next bar I! may be 4A, that of bar l6, 9A, etc. In an alternative arrangement the widths may be varied in a sinusoidal manner so that the width of the bar l2 will'be the maximum width of any of the bars and that of an imaginary bar at a distance d beyond I 8 will be zero, the distance between the centers of the imaginary bar and bar 12 corresponding to degrees of the sine function which determines the bar areas.

Referring to Fig. 4 transmitters T1, T2, T3 and T4 which may correspond in every respect to those.

of Fig. 2 are connected to the line i which terminates at a remote point in the electric wave to sound transducer which may be a loud-speaker 4 having a sound-producing diaphragm 5. Beyond the diaphragm is a transmission type compressional wave diffraction grating 20 positioned in the path of the sound beam in the diaphragm 5. The diffraction grating 20 serves to difierently diffract the four beams whose mid-frequencies are f1, f2, f3 and L; as indicated. A focus'sing lens 2! in every respect similar to the lens I of Fig. 2 may serve to focus the individual beams of the four receiving elements R R R R in the manner already described in connection with the supersonic wave responsive devices of Fig. 1.

As indicated in Fig, 5 the arrangement of the elements of the grating 20 corresponds in a general way to that of the elements of the grating B as shown in Fig. 3 with the central opening 22 of the transmission grating 20 corresponding to the central reflecting bar I2 of the reflecting grating 6 and with the successively adjacent openings 23, 24, 25 of the grating of Fig. 5 varying in width in the same manner as the reflecting bars l5, Ni, ii of the grating 6. The absorbing strips 26, 21, 28, etc. of the grating 20 may consist of any suitable material such as, for example, Vistanex (polymerized isobutylene of extremely high molecular weight), or the strips 26. 21, 28, etc. can be made of steel which will reflect the undesired energy away from the direction of the receiving elements R R R R The individual compressional wave responsive devices R R R and R may each be connected to its individual amplifier, demodulator and signal indicator as indicated at 29, 39 and 3!.

Fig. 6 discloses a modified form of transmission diffraction grating in which the central half of the structure consists of reflecting or absorbing bars 21' having a widthof The two outer quarters constituting the remainder of the structure consist of reflecting or absorbing bars 26, 28 each having a width of This distribution of di'firacting area-s yields highly directive diffraction effe 4 The effect W-hiclrth unequal a-r principle of appliifa-nts novel grating produbes may b upon consideration of the graph of Fig.- 7 in which directivity characteristics or responses at a particular frequency are plotted against the phase angle between energies emanating from prognmate elements of the diffraction grating. The solid line curve marked equal area grating-if indicatesthe relative energies at variousangles from the condition of phase coincidence to-thatof 360 degreessepa ration. In order to make clear the reason for-this we mayadvert once more to .the diagram of Fig. l. e p 1 'The ray ClP combines with'the' energy emanatingfrom 02 because the two are in phase agreemerit. For rays, of all other directions between 01F and Ss-there will be lack; of phase agreement and a red-ueed resultanti This situation'is portrayed in Fig. '2 inwhich the resultant intensities in various directions ar plotted-interms of phase difference between the energies emanating from crand oz. The directly-reversed raye S4 and S5 are in phase agreement. Hence the phase diiierence of these reversed rays is zero. The phase at point P of the ray all? is one full wavelength ahead of the ray just starting from 02 at the instant that the ray c1P has reached point P. Hence the phase difference is 360 degrees and the ray ClP indicates the direction of the first order diffraction. In Fig. '7 the solid line shows distribution of energy for a grating having equal area diffracting bars. At the zero degree phase position which exists between the reflected rays S4 and S5 the intensity of the reflected wave is indicated by the major lobe 3l. It will be noted thatthis lobe falls rapidly to a low magnitude but is closely followed by the minor lobes 32 and 33 which are of successively smaller magnitude. At the phase angle 360 degrees there is a major lobe 34 for the first order of diffraction. Corresponding minor lobes 35 and 35 slightly precede the major lobe. Throughout the intervening angular range between lobes 33 and 35 the envelope 3! of the lobe peaks falls to a minimum value approximately 33 decibels below the magnitude of lobes 3| and-34.

The broken line graph portrays theperfornn ance of a diffraction filter having fifty elements with the phase areas of the individual elements varied in accordance with the square law. The major lobes 38 and 39 at zero degree and 350 degree phase an les, respectively, are equal in intensity to the major lobes 3i and 3 50f the solid line graph. The first minor lobe ifi adjacent the zero phase position and the first-minor lobe M adjacent the 369 degree division are greatly re duced, their peaks lying some 27 decibels below the peak ofthe major lobe. Moreover, as indicated by the envelope :32 the minimum intensity occurring at the 180 degree phase angle is'more than 50 decibels below the peak of the major lobe. It is accordingly apparent that a very important increase in directional discrimination is attained by the use ofthe square law area of variation principle. This increase in discrimination may be employed to produce more effective filtering with a diffraction grating structure of a given superficial area and cost oiit'may be utilized to reduce the number of elements-and the cost of a diffraction grating filter with reference to the area and cost of a filter utilizing equal area elements.

Fig. 8 shows the results obtained with a diffractidn grating filter" in which the areas of the iridivi'dualelements' are vaneam accordancewith sine law. This graph is pio'aed'so that th'e'rnaf- J'or-1obs'44 -a-nd'45 may be compared lob'e's' 3-1 and 3'4 respectively of Fig; 7. The minor lobes mane 4*! have peak intensities of the order of 2 3 states less thanthe major lobes. This an rangeinenttherefore does not yield quite as high a discrimination between the major lobe and the first minor lobe tut it does have'the advantage that in the central portion of the range as; will be observedby referring to the envelope 4 8 the intensity falls to'a magnitude 60 decibels below that of the major lobe. Since it is possible asl i'as already been explained by a suitable design of'the filter to orient the diffracted beams to the most favorable angular position this sinusoidal distribtrticn' affords a means of securing a very high discrimination between the desired energy of the first order diffraction and undesired energy at a position or angle substantially removed therefrom; I

Fig. 9 discloses'a system in which therefiection V grating is given a concave conformation men:

able it to repla'ce both the plane diffraction grating of the preceding figures and the'conve'rging le'n's employed therewith. It is well known that in the case of light if a point source be positioned on a; circle having a radius of curvature 2 having a center at the centraipoint of a sphericalgrating' facing the-circle andh'aviiig" a radius of curvature R, the various frequency comptnents will be separated from each other bydiffraction and will" fall at difi'erentpoints on the circle'at which'the point source lies; See Wood's Physical Optics, New and Revised Edition 1928; pages 231 to" 236, inclusive.) This plil'lcip le is employee; in the system of Fig.9 in which Ti, 'I'z T3 and T4 represent transmitting sources similar to those'of the systems-liirevidusly'described; The transiri-itters a'r'e connectdtoa commonline I which terminates in a compressional wave emitting element 56 which: may be of piezoelectric type; A seriesof energy absorbing elements' R R "R R tuned respectively to the frequencies of transmitters T1; T2, T3 and T4- are" arranged; along the circufnfefence or a circle passing through the element and-having itscenter at thepoint 51. A spherical diffraction grating 52 having a radius of curvature equal to twice the distance between element 59 and point 5| is positioned facirigthe array of energy absorbing elements and withits centra-l mendber at the point-5i. The entire as: semblageofenergy radiator fifljenergy absorbing ele'nie'iitsR R Rf; and R and the spherical diffraction grating; 52=is enclosed within" a container -5-3' filled with a liquid such as castor oil. The con taine'r preferably eonsists of acoustic absorbing materialt'oprevent reflections from its; walls to the interiorelenfientsr It may consistof an outer shell 5'4 which isfluid'tight with'aninnerscreen 5 5 betw'e e'ii; which are retained copper shavings'or copper foil" saturated with castor oil. A baffle 56 is'introduce'd in direct'lindbetween the energy radiator 59 and theiene'rg'y absorbing devicesto prevent direct transfer ofcompressional'wave en'- ergy therebetween. :In operation compressional waves; set up in the ca'tor'oil' by thfc'ornprss'ional wave emitter 5 0 impinge umsmhe' concave reneaidrrtyri unnatural" atingsztana a'refdiffrac'ted" iii difierent dire'cti'onsf so that wares of the frequency originating" at transmitter "Ii' im' pinge upon the energy absorbing member R and wavesof the frequencies originating at each of the other transmitters likewise impinge upon a single respective energy absorbing member. As in the case of the preceding figures each of the energy absorbing members is provided with its individual amplifier, demodulator and signal indicator. In this system as in the preceding systems the principle of utilizing maximum efliciency reflection at the center with decreasing reflection toward the outer periphery is employed. The central reflector may therefore have the form of a concave disc. The remaining reflector elements comprise annuli of spherical contour which have equal spacings between their center lines and which decrease systematically in width in accordance with the sine law from the central member to the outer member. By this expedient the major lobe for the first order of diffraction is greatly accentuated in amplitude relative to the remaining lobes.

What is claimed is:

l. A diffraction grating for reflecting waves having substantially plane wave fronts to different angles according to their frequencies comprising a grill of parallel reflecting bars having flat reflecting surfaces so disposed with respect to each other as to constitute elements of a common larger surface, the central longitudinal axes of the bars being equally spaced, the refleeting surface area of the central bar being substantially equal to the area of each of the contiguous slots between it and the adjacent bars and the remaining bars having reflecting surfaces the widths of which progressively decrease from that of the central bar to that of the outermost bars in a substantially sinusoidal fashion.

2. A diffraction grating for separating diiferent frequency waves comprising parallel alternate transmission and non-transmission elements of substantially equal lengths, the central axes of the transmission elements being uniformly spaced and the transmission elements decreasing in area from the central element to the outer element in sinusoidal fashion with relation to distance from the center of the grating whereby the non-transmission elements increase sinusoidally from the center to the outermost element but in a converse manner.

3. The method of increasing the directive selectivity of a diffraction grating which comprises placing proximate elements so that the loci of their center points are substantially equidistant one from another, spacing the central diifractive elements from each other by about their own widths in order to obtain maximum primary order diffraction therefrom, and varying the widths of the dilfractive elements to cause them to progressively diminish from the center outwardly approximately in accordance with a square law relationship.

4. The method of increasing the directive selectivity of a diffraction grating comprising paralel elements of substantially equal lengths, the central longitudinal axes of the proximate elements being uniformly spaced, which consists in spacing the central elements by such distance as to give substantially maximum diffraction effects and varying the areas of diffractive elements more remote from the center to reduce them with reference to the areas of the central diifractive elements whereby the discrimination between the major lobe of a particular order of diffracted energy with reference to a minor lobe is augmented.

5. .A filter for compressional waves comprising a diffraction grating having parallel elements each provided with a face substantially aligned with corresponding faces of the remaining elements to constitute a striated surface, the separation of the central longitudinal axes of proximate elements being uniform, the widthof the central diifractive elements being substantially equal to the width of the spaces intervening and the width ofthe outer elements decreasin from a maximum at the central part of the structure in accordance with the square of the distance therefrom to a minimum at the outermost element.

6. A diffraction grating comprising a plurality of parallel elements having one face of each aligned with the corresponding faces of the remainin elements to constitute a striated surface upon which wave energy may fall, the elements having uniform spacing between the central longitudinal axes of proximate elements, the width of the aligned faces of the elements nearest the center of the structure being greater than that of the elements more remote from the center and the width of the faces of the series of elements decreasing progressively from the center to a minimum at the outermost element.

7. A multichannel selective system for supersonic waves comprising a source of supersonic waves of different frequencies, a difiraction grating positioned in the path of a beam of waves emanating fromsaid source, refractive means for focussing a plurality of different frequency components of a diffracted beam at respectively different points and individual compressional wave responsive devices positioned at each of said points but having energy receiving surfaces upon which the focussed diffracted beams may impinge.

8. A supersonic wave source comprising a supersonic wave energy emitting member having a substantially plane surface, the transverse dimensions of which are relatively high compared to the wave-length of the supersonic energy to be transmitted whereby a highly directive beam of supersonic energy may be emitted, a diffraction grating in the path of said beam comprising parallel elements, the central points of which are equally spaced throughout the series of elements but the widths of which progressively decrease from the central element to the outer elements, a converging lens positioned in the path of a diffracted beam from said grating, and a plurality of compressional wave responsive devices positioned respectively at the feel at which said lens causes a corresponding plurality of different predetermined frequency components of the diffracted beam to be focussed.

9. A diffraction grating comprising a plurality of parallel bars, said elements having one side of each so positioned as to constitute a striated surface, the thickness of the reflective bars thereof in the direction perpendicular to said surface corresponding to substantially onequarter wave-length in the medium of the reflective bars for waves of a predetermined frequency in combination with a wave-responsive device sensitive to said frequency, and so positioned with respect to said grating as to receive a maximum beam of said frequency diffracted therefrom.

10. A diffraction grating comprising a plurality of parallel flat bars having their corresponding major faces aligned in substantially the same planes, the faces in one of said planes being highly polished and the distance between said faces being substantially oneuarter wavelength the medium of the bars for Waves of a predetermined frequency, in combination with a wave-responsive device sensitive to said frequency, and so positioned with respect to said grating as to receive a maximum beam of said frequency diffracted therefrom.

1,1. A source of wave energy comprisin a means -for emitting waves of substantially one frequency, a difiraction grating positioned in the 10 path of a beam of Waves emanating from said source and having a plurality of parallel bars, said bars having one side of each so positioned as to constitute a striated surface, the thickness of the reflective bars thereof in the direction perpendicula-r to said surface corresponding substantially to one-quarter of the wave-length of the emitted frequency in the medium of the reflective bars,

WARREN P. MASON. 

